1. Field of the Invention
The present invention relates to an apparatus for washing substrates such as semiconductor wafers.
2. Description of the Related Art
The washing system has been used in the course of making semiconductor devices to remove particles, organic contaminating matters, metal impurities, naturally-formed oxide film and so on from surfaces of semiconductor wafers, for example. The washing system includes a plurality of substrates-washing apparatus and each of them includes a process vessel. The wafers are successively processed by ammonia, water fluoric acid, water, sulfuric acid, water, hydrochloric acid and water in these process vessels.
Each of the process vessels includes a wafer-holding boat and a straightening plate. A plurality of apertures are formed in lines in the straightening plate and the pitch interval of the apertures in one line is same. The boat can be carried into and out of the process vessel together with the wafers. When the wafers are immersed into washing solution in the process vessel, new washing solution is supplied into it through openings in its bottom to create a rising flow of washing solution in it. This rising flow of washing solution passes through the apertures in the straightening plate, contacts the wafers and overflows from the process vessel.
In the conventional apparatus, the pitch interval of the apertures in each line is same as that of the wafers held in the process vessel. As the wafers become larger in number, therefore, the number of the apertures in the straightening plate must be increased more and more. The straightening plate, however, is made of hard and breakable quartz. When the pitch interval of the apertures in each line becomes small, therefore, it is difficult to form the apertures in the straightening plate. When the diameter of each of the apertures is made small to meet this small pitch interval, the amount of washing solution passing through the apertures becomes insufficient and the time needed to wash the wafers is thus made long.
Further, the apertures are formed in all over the straightening plate in the conventional apparatus. However, those areas of the process vessel in which no wafer is present are quite larger than that area thereof in which the wafers are present, and washing solution flows more into the former areas but less into the latter area even though the apertures are formed in all over the straightening plate.
Jpn. Pat. Appln. KOKAI Publication No. Sho 58-61632 discloses a washing vessel provided with a straightening plate in which the apertures are formed to allow washing solution to fully flow to that area of the washing vessel which is remote from solution supply openings in the bottom of the vessel. Jpn. Pat. Appln. KOKOKU Application No. Sho 62-42374 also discloses a washing vessel wherein the diameter of each aperture in a first area of the straightening plate which corresponds to the center portion of the wafers is made large, the diameter of each apertures in second areas thereof which correspond to side portions of the wafers is made smaller than that of each aperture in the first area, and the diameter of each aperture in third areas thereof in which no wafer is present is made smaller than that of each aperture in the second areas.
In these straightening plates, however, the diameter of each aperture must be made different every time the area of the straightening plate in which the apertures are to be formed changes. This makes it troublesome to make the straightening plates. In addition, various kinds of apertures which are different in diameter must be formed in a large number in the straightening plate. The cost is thus made high.
Further, the circular wafers are carried into the rectangular vessel in the conventional substrates-washing apparatus. The volume of the washing vessel is therefore made so large as to create a dead space therein which does not contribute to washing the wafers.
Chemical solutions (such as hydrofluoric acid) are comparatively expensive. Any chemical solution overflowing from the washing vessel, therefore, is circulated. After it is contaminated to some extent, it is then exhausted as waste. When the volume of the washing vessel is large, the amount of this expensive chemical solution used is increased to thereby make the running cost high.
When the wafers which have have been processed by chemical solution are washed by pure water, this pure water introduced is not circulated but exhausted as waste at the time when it overflows from the vessel. The time needed to finish the washing of the wafers and the amount of pure water used in this manner are proportional to the volume of the washing vessel. The 8-inch wafers are recently used more and more instead of the 6-inch wafers and the washing vessel is thus made larger to meet these 8-inch wafers. It is therefore asked that a higher wafers-washing efficiency can be attained by a less amount of washing solution used.